The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A PN junction of a light emitting diode (LED) emits light when the PN junction is forward-biased. Generally, LEDs are energy-efficient, reliable, low-maintenance, and environmentally friendly. Accordingly, LED-based displays (luminaires) are used in a variety of residential and commercial applications. For example, the displays are used in microwave ovens, advertizing signs, industrial control panels, street lights, and so on.
Luminosity of LEDs is typically a function of a forward current through the PN junction when the PN junction is forward-biased. Additionally, the luminosity is a function of a temperature of the PN junction (junction temperature). A forward voltage applied across the PN junction determines the forward current through the PN junction. The forward voltage is also a function of the junction temperature.
Referring now to FIGS. 1-5, various characteristics of LEDs are shown. While the characteristics of LEDs manufactured by different manufacturers may vary slightly, the characteristics generally have similar templates. In FIG. 1, a graph of relative luminous flux (φv/φv(350 mA)) of an LED is shown as a function of forward current IF of the LED at a predetermined ambient temperature (e.g., TA=25° C.). As shown, at a predetermined ambient temperature TA, the relative luminous flux increases approximately linearly as the forward current IF increases.
In FIG. 2, a graph of a forward current IF of an LED is shown as a function of a forward voltage VF of the LED at a predetermined ambient temperature (e.g., TA=25° C.). As shown, at a predetermined ambient temperature TA, the forward current IF increases as the forward voltage VF increases.
In FIG. 3, a graph of a relative forward voltage (ΔVF=VF−VF(25° C.)) of an LED is shown as a function of a junction temperature Tj of the LED at a predetermined forward current IF (e.g., 350 mA). As shown, the relative forward voltage ΔVF to maintain a predetermined forward current IF decreases as the junction temperature Tj increases.
In FIG. 4, a graph of a relative luminous flux (φv/φv(25° C.)) of an LED is shown as a function of a junction temperature Tj at a predetermined forward current IF (e.g., 350 mA). As shown, at a predetermined forward current IF, the relative luminous flux decreases approximately linearly as the junction temperature Tj increases.
In FIG. 5, a table shows variations in forward voltage VF and relative luminous flux (RLF) of an LED over a wide temperature range (e.g., from −20° C. to 80° C.) at a predetermined forward current IF (e.g., 350 mA). As shown, the power to maintain consistent luminosity increases as the temperature increases.
In summary, while the forward current IF determines the luminosity of the LEDs, the forward current IF and the forward voltage VF that determines the forward current IF depend on temperature (i.e., the junction temperature Tj and the ambient temperature TA). Accordingly, the luminosity of the LEDs can change when the junction temperature Tj and the ambient temperature TA change. Specifically, at a predetermined forward current IF (or forward voltage VF), the luminosity decreases as the temperatures increase.
Additionally, due to die-to-die variations during manufacture, LEDs may exhibit different IF/VF characteristics. Further, the LEDs may exhibit different luminosities for the same forward current IF. Consequently, the light output of the LEDs may vary at the same temperature or within a temperature range. While variations in the light output may be tolerable in some applications, the variations may be unacceptable in commercial applications.